262 MORPHOLOGY AND EVOLUTION
in the female. These main sacs give off many diverti-cula which run in the same directions as those in the Gorilla. When the sacs are inflated they are seen to form a large balloon between the mandible above and the sternum below, with lateral offshoots passing round the mandible towards the ears. Very large diverticula pass into the axillae. In the Siamang the air-sac communicates with both laryngeal ventricles, and appears to be intermediate in character between those of Apes and those of the Monkeys.
The functions of the air-sacs in the Apes are not known, but it is almost certain that they are not entirely resonating chambers as in the Monkeys. They may however modify the voice and render it deep and sonorous. Some authors have ascribed to them the purely mechanical function of supporting the heavy jaws, and Vrolik (411) believes they are of great importance for this purpose in old age.
Trachea and Bronchi.*—The characters of these structures are similar in the Apes and Man.*
Lungs.—Many variations in the number of lobes into which the lungs are divided have been recorded. In the Gibbon, according to the observations of Kohlbriigge (199), Deniker (44) and myself, the left lung is divided into two lobes and the right one into four, of which one corresponds to the azygos lobe of the lower Mammalia. In the Gorilla the azygos lobe has only been observed in one animal described by Bischoff (425). In all other Gorillas, as in most Chimpanzees, the left lung is divided into two lobes and the right one into three. Sonntag (399) has
* Aeby (3) described the bronchi